The prevalence of wireless electronic devices has placed increasing constraint on the power performance of electronic circuits. The convenience and utility of battery powered wireless devices are greatly improved through the use of low power circuits. Circuit techniques that preserve power are, therefore, increasingly important in order for these devices to keep in step with higher consumer expectations for convenience and device functionality.
A common method of preserving power in electronic circuits involves placing the circuit into different phases of variant levels of power consumption and functionality. Such techniques may preserve power due to the inverse relationship of functionality and power consumption in electronic circuits. For example, a relay transmitter will wake up and transmit information in its regular operating state after receiving a stimulus, but will then return to a low power state while waiting for further instructions.
The time it takes for a circuit to transfer from a low power, low functionality state to a high power, operational state is referred to as the startup time. Modern circuits may make such transitions several times in a single millisecond. While the circuit is in transition, it is not operational, but it is still consuming more power than it does in its dormant state. In the interest of decreasing power consumption and improving performance, it is best to keep the transition time as short as possible. This is a second order form of power performance improvement that was marginalized in the recent past. Startup circuits were usually focused on reliability while speed was a secondary concern. With the tighter power constraints of wireless electronics, fast startup circuits have become increasingly valuable.
A clocking circuit comprises one of the base levels of functionality for a digital electronic circuit. A common form of clocking circuit utilizes a resonant element, such as a piezoelectric resonator or resonant micro-electromechanical system (MEMS) element, to set the frequency of the clock. The frequencies at which such resonators oscillate are referred to as the resonant frequencies. Each resonant element has its own resonant frequencies that are based on certain parameters such as the material and geometry of the element. During operation of the clocking circuit, a particular resonant frequency is selected and utilized depending upon the desired clock frequency. The resonant element is coupled to feedback circuitry that provides it with energy at the desired resonant frequency to maintain oscillation. Before the resonator begins to oscillate the feedback system is rudderless and has nothing to feedback to the resonator. Therefore, the resonator and accompanying feedback circuit must receive energy from an external source in order for oscillation to commence.
It is generally known that a crystal oscillator starts up when it is provided with energy from an external source. The energy from an external source places the crystal in a nonzero energy state from which the oscillator state evolves. The most efficient stimulus has a frequency equal to the resonant frequency as energy at other frequencies will be rejected by the oscillator. The initial solution for this problem was to configure the circuit so that is was sensitive to thermal noise during startup. Thermal noise is low energy white noise and is always present in circuits. White noise is random and covers all frequencies. Therefore, the crystal would receive energy across a wide bandwidth as well as at its resonant frequency without the circuit designer having to determine and target the resonant frequency. Circuits that applied this method involved placing the circuit into two different states. During startup the circuit would be placed in a first state that would allow a large amount of noise in the circuit. This high level of noise would decrease the startup time. After startup the circuit would transfer into a low noise state for proper operation.
Related support circuits were focused on determining when oscillation had begun so that the circuit could be switched into its operating state at the soonest possible time. These circuits would therefore act to minimize the startup time. The main drawback of these circuits is that power would be wasted since white noise has just as much energy at the target frequency as it does at other spurious frequencies. The low power of thermal noise also limited the energy delivered to the resonator which increased the required startup time.
Another technique involves leaving the crystal in oscillation during shutdown and avoiding the need to start it up again altogether. An example of this technique can be found in U.S. Pat. No. 5,155,453. Circuits utilizing this method report impressive startup times of 0.25 milliseconds. However, this technique is not fairly comparable to full shutdown techniques because power consumption in the shutdown state is necessarily higher for circuits that maintain oscillation.
As power consumption has fallen under greater scrutiny, more complex circuits have been developed that generate specific impetus signals rather than relying on random noise. A single pulse impetus signal is presented in U.S. Pat. No. 5,805,027 by Yin. Performance figures were reported in the range of 100 milliseconds. Another similar technique described in U.S. Pat. No. 6,057,742 by Prado involved the use of a noise pulse impetus signal. This method has the benefit of roughly targeting the specific resonant frequency of the crystal while not interrupting the bias point of the circuit as would occur from a single large step pulse.
Recent circuits have begun to more actively target the impetus signal that will produce optimal startup. In a circuit developed by Gazit in U.S. Pat. No. 7,009,458 a train of pulses is provided to the piezoelectric resonator by a second oscillator with a short start up time. A problem that arises with impetus signals of this form is that the circuit needs to shut them off soon after oscillations are triggered. If the impetus signal is not disconnected the circuit may suffer from resonator overdrive. Resonator overdrive is similar to the effect that an extremely large push has on a rope swing. If one pushes on an empty swing as hard as they can the ropes will go slack and the swing will not continue swinging.
The method proposed by Gazit does not deal with targeting the specific resonant frequency of the piezoelectric oscillator. The method instead focuses on the optimal number of pulses provided to the resonator before problems with resonator overdrive occur. This circuit requires a counter circuit to be active during startup to measure the number of pulses delivered to the resonator. This adds to the circuit's power consumption during startup. In addition, the optimal number of pulses will depend on the particular resonator on a device to device basis and will not likely provide enough resolution to trigger the optimal startup time for any given resonator.
Another circuit that uses a multiple pulse impetus signal is that developed by Blanchard in U.S. Pat. No. 6,819,195. The impetus signal in this circuit is targeted at and is calibrated to the resonant frequency of the resonator. Calibration is accomplished by tuning the bias current of a ring oscillator. Control circuitry controls the time for which such a signal is applied so that oscillations begin under all operating conditions. The main oscillator is coupled to the impetus oscillator through two transconductance amplifiers that provide AC current to the two terminals of a differential oscillator.
The Blanchard circuit shares the problem addressed by the Prado circuit. Since the impetus signal is DC coupled to the oscillator the bias points of the oscillator circuit will be severely affected during the startup phase. Once the impetus signal is removed, there will be an added transient period required while the circuit adjusts to the correct DC operating point. This increases the overall startup time. The Blanchard circuit also suffers from the drawback of resonator overdrive in that the impetus signal is DC coupled to the main oscillator terminals. This drawback necessitates the use of complex control and monitor circuitry that turns off the impetus signal at a particular time. The Blanchard circuit is also differential which necessitates the use of two buffers. Since buffers are generally the most power hungry devices in an oscillator circuit, the power consumption of the circuit during startup is not optimal.
The susceptibility of prior art to resonator overdrive and self cancelling is revealed by the simplified circuit diagram shown in FIG. 1. The AC current source buffers 102 deliver the impetus signal to piezoelectric oscillator 104. Oscillator 104 is known as an inverter oscillator and is composed of piezoelectric resonator 106, two capacitors 105 and 111, resistor 107 and inverting amplifier 109. Buffer 110 delivers the output oscillation signal of piezoelectric oscillator 104 to compare and control circuitry 103. The piezoelectric oscillator feedback circuit is driven by amplifier 109. The impetus signal and the piezoelectric oscillator's output signal are DC coupled and drive the same nodes in the circuit. Therefore, if the tunable oscillator contains spurious frequencies, such signals are applied by powerful buffers that directly conflict with the piezoelectric oscillator.
Sensitivity to overdrive and self cancelling requires the use of complex monitor and control circuitry that assures the impetus signal generator is deactivated after a stable operational point has been reached. The prior art managed this requirement through the use of a counter circuit in digital circuitry 103 that counted the number of pulses provided by tunable oscillator 101 to piezoelectric oscillator 104. After a certain number of pulses were delivered to the piezoelectric oscillator the AC current source buffers 102 would be disabled by the compare and control circuitry 103. The added complexity of this circuit affects reliability. In addition, the added circuitry increases the cost and power consumption of the circuit.
The circuit's method of driving the oscillator has the additional drawback of affecting the DC bias points. In some oscillators the nodes that are driven by the impetus signals will need to be biased at specific DC values during operation. When enabled, AC current source buffers 102 will be driving these nodes to values that are far from their DC bias. It will take additional time after the impetus signal is disabled for the circuit to adjust from the driven bias point to the correct operating state DC bias point. The distance these points drift will be random, and a circuit designer will have to wait for the worst case time period before operating the circuit. This is an undesirable condition since power will be wasted while the circuit is settling.